1. Field of the Invention
The present invention relates to a surface acoustic wave filter apparatus including a plurality of multi-mode filters which are defined by edge-reflection type surface acoustic wave devices, and more particularly, to a surface acoustic wave filter, a duplexer and a communications apparatus which has a very small size and can be connected with a minimum bonding wire because of an improved arrangement and structure of the multiple multi-mode filters.
2. Description of the Related Art
Conventionally, a multi-mode filter including a plurality of edge-reflection type surface acoustic wave resonators has been used as a bandpass filter in a portable communications apparatus such as a cellular phone. A transversally coupled multi-mode filter wherein edge-reflection type surface acoustic wave resonators, which operate using SH type surface acoustic waves such as Love waves and piezoelectric surface shear waves, are electrically connected in series, and a longitudinally coupled multi-mode filter, wherein edge-reflection type surface acoustic wave resonators are acoustically arranged in series, are known types of such a multi-mode filter.
These types of multiple multi-mode filters are often connected in multiple stages to improve the passband width and attenuation characteristics thereof.
FIG. 8 is a plan view of an example of a conventional surface acoustic wave filter, in which three stages of transversely coupled multi-mode filters are electrically connected in series.
The surface acoustic wave filter 101 includes transversely coupled multi-mode filters 102, 103 and 104. As shown in FIG. 8, the three multi-mode filters 102, 103 and 104 are electrically connected in series between input/output ports 105 and 106 by using bonding wires 107.
Each of the multi-mode filters 102, 103 and 104 includes two interdigital electrode transducers (hereinafter referred to as "IDT") disposed on piezoelectric substrates 102x, 103x and 104x, respectively, to define two edge-reflection type resonators. Generally, an IDT has a pair of comb-shaped electrodes which include a plurality of electrode fingers and a bus bar connected to one end of the electrode fingers, respectively. In addition, the pair of comb-shaped electrodes are arranged such that the electrode fingers of comb-shaped electrodes are interdigitated with each other.
Note that, however, in each of the multi-mode filters 102, 103 and 104, one of the comb-shaped electrodes in a first of the IDTs and one of the comb-shaped electrodes in a second of the IDTs are arranged so as to share one bus bar or so as to define one integrated bus bar. As a result, each of the multi-mode filters 102, 103 and 104 actually has three comb-shaped electrodes. That is, two comb-shaped electrodes are arranged such that the bus bars thereof are parallel to each other and such that the electrode fingers thereof respectively extend inward between the bus bars so as to interpose a comb-shaped electrode which includes one bus bar located between the two comb-shaped electrodes and electrode fingers on both sides of the bus bar so as to extend toward the two comb-shaped electrodes, respectively. As a result of this structure, two edge-reflection type resonators of each multi-mode filter are electrically connected in series. When a signal is input to one IDT of each multi-mode filter, a standing wave is generated and an output signal based on the standing wave is output from the other IDT. Since the higher order mode of the standing wave having a different resonance frequency is also generated, each multi-mode filter exhibits filter characteristics.
FIG. 9 is a plan view of an example of a conventional surface acoustic wave filter, in which longitudinally coupled multi-mode filters are electrically connected in series in two stages.
The surface acoustic wave filter apparatus 111 has longitudinally coupled multi-mode filters 112 and 113. As shown in FIG. 9, the two multi-mode filters 112 and 113 are electrically connected in series between input/output ports 115 and 116 via a bonding wire 117.
Each of the multi-mode filters 112 and 113 include two IDTs disposed on piezoelectric substrates 112x and 113x, respectively. Each of the IDTs has a pair of comb-shaped electrodes which include a plurality of electrode fingers and a bus bar connected to one end of the electrode fingers, respectively. In addition, the pair of comb-shaped electrodes are arranged such that the electrode fingers of comb-shaped electrodes are interdigitated with each other.
In each of the multi-mode filters 112 and 113, the two IDTs are arranged in the direction in which the surface acoustic wave propagates. When a signal is input to one IDT of the each multi-mode filter, a standing wave is generated and an output signal based on the standing wave is output from the other IDT. Since the higher order mode of the standing wave having a different resonance frequency is also generated, each multi-mode filter exhibits filter characteristics.
In the conventional surface acoustic wave filters 101 and 111, the multi-mode filters 102.about.104, 112 and 113 are provided on separate piezoelectric substrates 102x.about.104x, 112x and 113x. Therefore, each of the multi-mode filters 102.about.104, 112 and 113 must be connected with each other using bonding wire, which makes the manufacturing process complicated.
Further, the end surfaces of the piezoelectric substrate in the edge-reflection type resonators must be formed with high precision in order to achieve accurate operation of the edge-reflection type resonators. Since these end surfaces are formed by cutting piezoelectric substrates via dicing, the dicing process must be carried out for each piezoelectric substrate with high precision. As a consequence, dicing must be carried out many times and very carefully, making the manufacturing process complex.
In addition, the piezoelectric substrates are located at a fixed distance from each other in order to prevent mutual electromagnetic interference between the multi-mode filters, which makes it difficult to produce a small-size surface acoustic wave filter apparatus.